In general, a wind turbine or a wind farm is operated with the aim to obtain maximum yield of the capital invested in it, and consequently the wind turbine control systems are configured to maximize the output power, i.e. to operate the wind turbine to capture the maximum power that is available in the wind, with due regard to keeping the wind turbine within load limits.
Nevertheless, there may be situations where there is a desire to operate the wind turbine in a manner where the requested output power is actually lower than an available power in the wind. This is referred to as derated operation, curtailed operation, and other terms are also used in the art. Derated operation may be desired for a number of reasons, one being an interest in having a power buffer which may be injected into the grid, e.g. in connection with frequency control.
Derated operation may be done in a number of ways, and a wind turbine may even implement more than one method, in particular one method may be used when operating in full load where the wind allows for full power production, and another method may be used in partial load where the wind only allows for partial power production, as compared to the rated power production of the wind turbine. For a wind turbine with variable pitch, this relates to that in partial load, the primary control is based on speed control by adjusting a load torque provided to the rotor by the generator coupled to the rotor, whereas in full load the primary control is based on active pitching.
WO 2010/000648 A2 discloses a method of operating a wind turbine in curtailed mode, where in partial load a curtailment is obtained, by offsetting the pitch value from an optimal pitch value to thereby capture less energy from the wind than what is available.
However, there is general a need in the art to provide further and improved ways of operating wind turbines in a derated manner.